Method of treating a titanium anode

ABSTRACT

1. A METHOD FOR THE ELECTRODEPOSITION OF MANGANESE DIOXIDE WHICH COMPRISES: PRETREATING A TITANIUM ANODE BY PROVIDING AN AQUEOUS ELECTROLYTE CONTAINING AT LEAST 25 GM./L. FLUORIDE IONS AND FROM ABOUT 800 TO 1200 GM./L. OF AT LEAST ONE OTHER COMPOUND SELECTED FROM THE GROUP CONSISTING OF ACETIC ACID, ETHYLENE GLYCOL AND A MIXTURE OF NITRIC ACID AND PHOSPHORIC ACID; PLACING SAID TITANIC ANODE IN CONTACT WITH SAID ELECTROLYTE; PLACING A CATHODE IN CONTACT WITH SAID ELECTROLYTE; ELECTROLYZING SAID ELECTROLYTE AT AN ANODIC CURRENT DENSITY OF AT LEAST 10 AMPS/FT,2; PLACING THE TREATED TITANIUM ANODE IN AN ELECTROLYTIC CELL FOR THE ELECTRODEPOSITION OF MANGANESE, SAID CELL CONTAINING AN ELECTROLYTE COMPRISING AN AQUEOUS SOLUTIONS OF MANGANOUS ION IN A CONCENTRATION OF FROM ABOUT 15 TO 50 GM./L. AND FROM ABOUT 10 TO 25 GM./L. SULFURIC ACID, EFFECTING THE ELECTRODEPOSITION OF MANGANESE DIOXIDE ON THE ANODE AT AN ANODIC CURRENT DENSITY OF FROM ABOUT 8 TO 25 AMP/FT.2 AND RECOVERING SAID ELECTRODEPOSITED MANGANESE DIOXIDE.

United States Patent O 3,841,978 METHOD OF TREATING A TITANIUM ANODESan-Chang Lai, Edmond, Okla., assignor t Kerr-McGee Chemical Corp.,Oklahoma City, Okla. No Drawing. Filed Dec. 11, 1972, Ser. No. 313,851Int. Cl. C01b 13/14 U.S. Cl. 20429 3 Claims ABSTRACT OF THE DISCLOSUREThe present invention relates to a method of treating a titanium anode.Broadly, the method comprises anodically treating the titanium anode inan electrolyte containing fluoride ions.

BACKGROUND OF THE INVENTION It is known to produce electrolyticmanganese dioxide from an electrolyte comprising a manganesesulfatesulfuric acid solution. The manganese dioxide is deposited on theanode under carefully controlled conditions of electrolysis to produce adeposit having certain desired properties. The manganese dioxide soproduced is recovered and used, for example, as a depolarizer forbatteries.

When graphite is used, as the anode in such an electrodeposition processthe current efliciency generally is limited to less than about 90%.Further, the manganese dioxide is strongly attached to the graphitesurface and subsequent removal of the manganese dioxide also results inremoval of particles of graphite, which necessarily contaminate themanganese dioxide. Moreover, it is diflicult to remove the depositedmanganese dioxide without damaging the graphite anode.

Certain other materials also have been suggested for use as anodes inthe electrodeposition of manganese dioxide, such as, for example,platinum, platinum plated titanium and other noble metals and theiroxides. The cost of such anodes is too high for an economical commercialoperation.

In U.S. Pat. No. 2,608,531 there is suggested a process for preparingelectrolytic manganese dioxide using a particular type of titaniumanode. Specifically, the patent discloses an anode consistingessentially of titanium particles compacted into a coherent butincompletely consolidated porous mass. However, such an anode is weak inmechanical characteristics and subject to breakage.

More recently, in U.S. 3,436,323 it is suggested that the disadvantagesof a compacted titanium anode can be overcome by using an anode having afinely and sharply indented aventurine surface. Such surface preferablyis obtained by sand blasting the surface of the anode. It has beenfound, however, when using such an anode, that an anodic current densityin excess of about 9 amps per square foot results in passivation of thetitanium anode surface.

Passivation is indicated by an increase in the electrical resistance ofthe anode. An increase in resistance necessarily requires an increase inthe voltage applied to the anode to maintain a constant current densityand also results in a corresponding increase in power consumption.

Titanium is substantially impervious to most corrosive environments(such as electrolytes), however, its susceptibility to passivation haslimited its acceptance as a suitable anode material. Obviously,therefore, there is need for a method of treating titanium to reduce itssusceptibility to passivation.

SUMMARY OF THE INVENTION A method now has been discovered of treatingtitanium whereby it is suitable for use as an anode, for example, in theelectrodeposition of manganese dioxide. In accordance with the presentmethod the titanium to be treated is placed in an electrolyte comprisingan aqueous solution containing fluoride ions and at least one compoundselected from the group consisting of ethylene glycol, acetic acid and amixture of phosphoric acid and nitric acid. Thereafter, the titanium isanodically treated at an anodic current density of at least 10 amps persquare foot. It has been found that titanium treated in accordance withthe present method can be used as an anode in the electrodeposition ofmanganese dioxide at current densities of up to about 30 amps per squarefoot. Further, after repeated electrodepositing and stripping, thesurface does not passivate or show any evidence of corrosion.

DESCRIPTION OF THE PREFERRED EMBODIMENT In accordance with the presentinvention the titanium to be treated is made the anode of anelectrolytic cell which also contains a suitable cathode. The cathodematerial is not critical and suitable materials include copper, nickel,mild steel, stainless steel, graphite, carbon and the like.

The cell contains an electrolyte comprising an aqueous solution offluoride ions and at least one compound selected from the groupconsisting of ethylene glycol, acetic acid and a mixture of phosphoricacid and nitric acid.

The term titanium as used herein includes not only titanium but itsalloys such as, for example, Ti-13V-11Cr- 3A1, Ti-6Al-6V-2Sn andTi-8Al-lMo-1V. Also contemplated herein are metals selected from thegroup consisting of tantalum, zirconium, niobium, hafnium and theiralloys.

The titanium may be in any form such as, for example, bar, plate, flatsheet, sheets of expanded metal and the like. A preferred form forsubsequent use in the electrodeposition of manganese dioxide isdisclosed in U.S. Pat. No. 3,654,102.

The composition of the electrolyte is critical. The concentration offluoride ion in the electrolyte must be at least about 25 grams perliter of electrolyte. Generally, it is desirable that the fluoride ionconcentration be maintained within a range of from about 55 to 75 gramsper liter. Particularly good results have been obtained with a fluorideconcentration of about grams per liter.

The source of fluoride ion is not critical. The fluoride may beintroduced into the electrolyte in the form of an aqueous solution ofhydrofluoric acid, by bubbling fluorine gas through the electrolyte andthe like. Other sources of fluoride ion include sodium fluoride,potassium fluoride, magnesium fluoride, Zinc fluoride, trifluoaceticacid and other inorganic and organic fluoride compounds. Generally, itis preferred to use an aqueous solution of hydrofluoric acid as thesource of fluoride ion.

Further, the electrolyte also must contain at least one other compound,either acetic acid, ethylene glycol or a mixture of phosphoric andnitric acid. Such other compound should be present in an amount withinthe range of from about 700 to 1,300 grams per liter of electrolyte andpreferably within the range of from about 800 to 1,200 grams per liter.The optimum concentration will depend, among other things, on theparticular compound selected. When using ethylene glycol or acetic acid,it generally is preferred to maintain a concentration of from about 850to 1,000 grams per liter.

When using a mixture of phosphoric and nitric acid, however, it isadvantageous to maintain the concentration within the range of fromabout 8001,200 grams per liter and preferably within the range of fromabout 9001,100 grams per liter. Further, when the other compound is amixture of phosphoric and nitric acid, the weight ratio of phosphoric tonitric acid should be within the range of 5:1 to 2:3 and preferablywithin the range of from 3:1 to 3:2.

The actual part played by the fluoride and the other compound in thistreating method is not fully understood and the inventor does not wishto be bound by any particular theory. However, it has been found thatwhen titanium is treated in accordance with the present method, it maybe used in a manganese dioxide electrodeposition process at highercurrent densities and for longer periods of time, without passivationoccurring, than would otherwise be possible.

The titanium, after being placed in the electrolyte, is treated at ananodic current density of at least about amps/ftF. At lower currentdensities substantially no beneficial effect is obtained. The uppercurrent density limit is primarily a function of the composition of theelectrolyte. If too high a current density is applied the titanium will,of course, passivate; preventing elfective treatment. The optimumcurrent density for treatment with a specific electrolyte composition isreadily determinable experimentally. Generally, the anodic currentdensity is within the range of from about 10 amp/ft. to 30 amp/ft. andpreferably within the range of from about to 25 amp/ ft.

The treatment time generally is within the range of from about 2 minutesto about 30 minutes. As those versed in the art will appreciate, thetreatment time will be a function of the current density applied duringtreatment. More particularly, with a given electrolyte, a low currentdensity will require more time for an effective treatment than a highercurrent density. During the treatment a gray film forms on the surfaceof the anode. If the treatment is continued for an excessive length oftime the film will begin to peel or flake olf the anode.

Temperature does not appear to be a critical parameter in the treatmentmethod of the present invention. Indeed, within a temperature range offrom about C. to about 50 C. substantially no dilference in the efiicacyof the invention is observed.

After the titanium has been treated in accordance with the presentmethod, it then is removed from the electrolytic cell and preferablywashed with water prior to its use in an electrolytic cell. Titaniumtreated in accordance with the present invention is particularly usefulin the electrodeposition of manganese dioxide.

More particularly, the treated titanium is used as the anode in anelectrolytic cell for the electrodeposition of manganese dioxide. Thecell contains an electrolyte comprising an aqueous solution containingmanganous ion in a concentration of from about 15 to 50 grams per literand from about 10 to grams per liter sulfuric acid. The electrolytetemperature is maintained within a range of from about 90 to 98 C. Whenthe titanium anode has been treated in accordance with the method of theinstant invention it is possible to operate the cell at an anodiccurrent density of from about 8 to 25 amp/ft. or even higher. Generally,it is preferred to maintain the anodic current density Within the rangeof from about 12 to 20 amp/ftfi.

The following examples are set forth for the purpose of illustrationonly and are not to be construed as limiting the scope of thisinvention.

EXAMPLE I An untreated strip of titanium is obtained and placed in anelectrolytic cell as the anode. The electrolyte in the cell comprises800 ml./l. H PO (85%), 150 ml./l. HNO (71%) and 50 ml./l. (49%). Anelectric current is caused to flow through the titanium anode in anamount suflicient to provide an anodic current density of about 20'amp./ ft. for two minutes. The treated titanium then is removed andrinsed with water.

The treated titanium then is used as the anode in a manganese dioxideelectrodeposition process. During electrodeposition of manganese dioxideon the anode, the initial concentration of manganous ion (Mn++) and H 80in the electrolytfi is about 20 g./l. and 15 g./l. respectively.

The initial cell voltage to maintain an anodic current density of 16amp/ft. is about 2.0 volts. Manganese dioxide is electrodeposited on theanode for about 96 hours. During the electrodeposition the concentrationof manganous ion and H SO is maintained within the range of about 19 to24 g./l. and 15 to 26 g./l. respectively. At the end of that time thecell voltage required to maintain the current density at 16 amp/ft. isonly 2.5 volts. Thus after 96 hours of operation the cell voltage hasincreased only 0.5 volts. When the foregoing process is repeated usingan untreated titanium anode, the cell voltage increases 3.8 volts after96 hours.

This example clearly demonstrates the eflicacy of the present inventionwhen the treating electrolyte comprises an aqueous solution of fluorideions and a mixture of nitric and phosphoric acid.

EXAMPLE )1 An untreated strip of titanium is obtained and placed in anelectrolytic cell as the anode. The electrolyte in the cell comprises820 ml./l. ethylene glycol, 60 ml./l. H 0 and 120 ml./l. HF (49%). Anelectric current is caused to flow through the titanium anode in anamount suflicient to provide an anodic current density of about 20amp/ft. for three minutes. The treated titanium then is removed andrinsed with water.

The treated titanium then is used as the anode in a manganese dioxideelectrodeposition process. During electrodeposition of manganese dioxideon the anode, the concentration of manganous ion (Mn++) and H in theelectrolyte is maintained at about 25 g./l. and 20 g./1. respectively.The initial cell voltage to maintain an anodic current density of 16amp/ft. is about 1.8 volts. After 24 hours of electrodeposition theanodic current density is increased to 20 amp/ft? and theelectrodeposition continued for an additional 66 hours.

Manganese dioxide is electrodeposited on the anode for a total time ofhours. At the end of that time the cell voltage necessary to maintainthe 20 amp/ft. current density is only about 2.25 volts. Thus after 90hours of operation the cell voltage has increased only about 0.45 volts.When the foregoing process is repeated using an untreated titaniumanode, the cell voltage increases 6 volts after an hour, indicatingpassivation of the untreated titanium anode.

This example clearly demonstrates the efficacy of the present inventionwhen the treating electrolyte comprises an aqueous solution of fluorideions and an ethylene glycol.

EXAMPLE III Two untreated strips of titanium are obtained and placed inan electrolytic cell as anodes. The electrolyte in the cell comprises888 ml./l. glacial acetic acid and 112 ml./l. HF (49%). An electriccurrent is caused to flow through the titanium anodes in an amountsufiicient to provide an anodic current density of about 16 amp/ft Afterabout ten minutes one of the treated titanium strips (designated A) isremoved and rinsed with water. After an additional 10 minutes the othertitanium strip (designated B) is removed and rinsed with water.

The treated titanium strips A and B then are used as anodes in amanganese dioxide electrodeposition process. During electrodeposition ofmanganese dioxide on the anode, the concentration of manganeous ion(Mn++) and H 80 in the electrolyte is maintained at about 25 gm./l. and20 gm./l. respectively. The initial cell voltage to maintalil an anodiccurrent densty of about 20 amp/ft. is 1.8 v0 ts.

Manganese dioxide is electrodeposited on the anodes for a total time of168 hours. At the end of that time the cell voltage necessary tomaintain the 20 amp/ft. current density for strip A is about 3.4 voltsand for strip B only about 2.8 volts. Thus after 168 hours of operationthe cell voltage has increased only 1.6 and 1.0 volts for strips A and Brespectively. When it is attempted to repeat the foregoing process usingan untreated titanium anode, the cell voltage increases over volts afterless than an hour.

This example clearly demonstrates the eflicacy of the present inventionwhen the treating electrolyte comprises an aqueous solution of fluorideions and acetic acid.

The foregoing description and examples are intended to be illustrativeonly and are not to be construed as limiting the scope of the invention;reference being had to the appended claims for such latter purpose.

What is claimed is:

1. A method for the electrodeposition of manganese dioxide whichcomprises:

pretreating a titanium anode by providing an aqueous electrolytecontaining at least 25 gm./l. fluoride ions and from about 800 to 1200gm./l. of at least one other compound selected from the group consistingof acetic acid, ethylene glycol and a mixture of nitric acid andphosphoric acid;

placing said titanium anode in contact with said electrolyte;

placing a cathode in contact with said electrolyte;

electrolyzing said electrolyte at an anodic current density of at least10 amps/ftF; placing the treated titanium anode in an electrolytic cellfor the electrodeposition of manganese, said cell containing anelectrolyte comprising an aqueous solution of manganous ion in aconcentration of from about to 50 gm./l. and from about 10 to 25 gm./l.sulfuric acid, etfecting the electrodeposition of manganese dioxide onthe anode at an anodic current density of from about 8 to amp/ft. andrecovering said electrodeposited manganese dioxide.

2. The method of claim 1 wherein the anodic current density during thetreatment is maintained within the range of from about 10 amps/ft. toamps/ft 3. The method of claim 2 wherein the concentration of fluorideions is maintained within a range of from about to gm./l.; the selectedcompound is acetic acid present in an amount within the range of fromabout 850 to 1000 gm./l. and said anodic current density duringtreatment is maintained within the range of from about 15 to 25 amps/ftReferences Cited UNITED STATES PATENTS 3,455,798 7/1969 Mehne et al.20496 3,436,323 4/ 1969 Shimizv et al. 20496 3,616,279 10/1971 Kendall20456 R FOREIGN PATENTS 23,813 8/1970 Japan 20496 OTHER REFERENCESBureau of Mines Electrodeposition Cramer et 25 al., September 1967, pp.4 and 50.

RICHARD L. ANDREWS, Primary Examiner U.S. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CURRECTION Patent No. 8.41 r978 D t d October 15 1974 Inven San-Chenq'Lai It is certified that errorappears in the above-identified patent and that said Letters Patent arehereby corrected as shown below:

Column 3, line 66, after "50 ml./l." please insert HF Column 4, line 68,"densty" should read density Signed and sealed this 3rd daydof December1974.

(SEAL) Attest: v

McCOY M. GIBSON JR. 0. MARSHALL DANN Attesting Officer Commissioner ofPatents ORM PO-1050 (10-69) I USCOMM DC 60376P69 it u,s. eovzmmsm Pmm'msOFFICE: I969 o-ass-su

1. A METHOD FOR THE ELECTRODEPOSITION OF MANGANESE DIOXIDE WHICHCOMPRISES: PRETREATING A TITANIUM ANODE BY PROVIDING AN AQUEOUSELECTROLYTE CONTAINING AT LEAST 25 GM./L. FLUORIDE IONS AND FROM ABOUT800 TO 1200 GM./L. OF AT LEAST ONE OTHER COMPOUND SELECTED FROM THEGROUP CONSISTING OF ACETIC ACID, ETHYLENE GLYCOL AND A MIXTURE OF NITRICACID AND PHOSPHORIC ACID; PLACING SAID TITANIC ANODE IN CONTACT WITHSAID ELECTROLYTE; PLACING A CATHODE IN CONTACT WITH SAID ELECTROLYTE;ELECTROLYZING SAID ELECTROLYTE AT AN ANODIC CURRENT DENSITY OF AT LEAST10 AMPS/FT,2; PLACING THE TREATED TITANIUM ANODE IN AN ELECTROLYTIC CELLFOR THE ELECTRODEPOSITION OF MANGANESE, SAID CELL CONTAINING ANELECTROLYTE COMPRISING AN AQUEOUS SOLUTIONS OF MANGANOUS ION IN ACONCENTRATION OF FROM ABOUT 15 TO 50 GM./L. AND FROM ABOUT 10 TO 25GM./L. SULFURIC ACID, EFFECTING THE ELECTRODEPOSITION OF MANGANESEDIOXIDE ON THE ANODE AT AN ANODIC CURRENT DENSITY OF FROM ABOUT 8 TO 25AMP/FT.2 AND RECOVERING SAID ELECTRODEPOSITED MANGANESE DIOXIDE.